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# Hubble’s Law

Absorption lines of a galaxy shift toward the blue end of the spectrum when it moves toward Earth. The lines shift to the red end of the spectrum when a galaxy moves away from Earth. Hubble’s Law.

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## Hubble’s Law

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1. Absorption lines of a galaxy shift toward the blue end of the spectrum when it moves toward Earth. • The lines shift to the red end of the spectrum when a galaxy moves away from Earth.

2. Hubble’s Law • The Doppler effect can be used to determine how fast stars or galaxies are approaching/moving away from Earth • When a star or galaxy is approaching Earth, its spectrum lines are shifted toward the shorter (bluer) wavelengths. • When the star or galaxy is moving away, its spectrum lines shift toward the longer (redder) wavelengths. • The larger the observed shift, the greater is the speed. • Edwin Hubble discovered that light from most galaxies undergoes a red shift • Showed that nearly all galaxies are getting farther away from Earth.

3. Hubble’s Law • Hubble’s Law: the speed at which a galaxy is moving away is proportional to its distance from us • Space between galaxies is expanding in all directions • The universe as a whole is becoming larger • To visualize the movement of galaxies, imagine the dough for a loaf of raisin bread as it rises. • Raisins = individual galaxies or groups of galaxies • Dough = the universe • Distance between raisins increases, but they stay the same size • Most distant galaxies are moving away from us more quickly

4. Hubble’s Constant • Hubble’s constant is one of the most important and debated numbers in astronomy: • Ratio of velocity to distance in Hubble’s Law • Can be estimated by finding slope of a graph of velocity vs distance for a set of galaxies. • Expresses how fast the universe is expanding • Can be used to estimate the age of the universe

5. The Big Bang Theory • Astronomers theorize that the universe came into being at a single moment, in an event called the big bang. • Motion of galaxies indicates that the universe is expanding uniformly. • The big bang theory states that the universe began in an instant, billions of years ago, in an enormous explosion. • The existence of cosmic microwave background radiation and the red shift in the spectra of distant galaxies strongly support the big bang theory.

6. The Big Bang Theory After the Big Bang The universe expanded and cooled down after the big bang. • After a few hundred thousand years of expansion, the universe was cool enough for atoms to form. • Gravity pulled atoms together into gas clouds that eventually evolved into stars in young galaxies. • The sun and solar system formed about 4.6 billion years ago, when the universe was about two thirds of its present size.

7. The Big Bang Theory Big bang occurred 13.7 billion years ago. • The universe began with the big bang about 13.7 billion years ago. • The first stars and galaxies formed 200 million years later. • The solar system, and Earth, formed about 9 billion years after the big bang. First stars and galaxies formed 200 million years after big bang. Solar system formed 4.6 billion years ago. Earth today

8. The Big Bang Theory Evidence for the Theory In 1965, Arno Penzias and Robert Wilson, using a radio telescope, noticed a faint distant glow in every direction. • This glow is called the cosmic microwave background radiation. • The glow is energy produced during the big bang, still traveling throughout the universe.

9. The Big Bang Theory • The big bang theory describes how the expansion and cooling of the universe over time could have led to the present universe of stars and galaxies. • It offers the best current scientific explanation of the expansion of the observable universe. • Variations of the theory continue to be proposed and are being tested with new observations.

10. The Big Bang Theory Age of the Universe • Since astronomers know how fast it is expanding now, they can infer how long the universe has been expanding. • If you traveled backward in time, all of the matter in the universe would be at its starting point about 13 - 14 billion years ago. • Recent measurements of the microwave background radiation have led to a more precise age. • Astronomers now estimate that the universe is 13.7 billion years old.

11. Continued Expansion • To have a gravitational force strong enough to reverse the expansion, there must be sufficient mass in the universe. • If there is less than this amount, the universe will continue expanding. • Much matter in the universe can’t be seen by astronomers. • Dark matter: matter that does not give off radiation • Cannot be seen directly • Can be detected by observing gravitational effects on visible matter • Galaxies like ours may contain as much as 10 times more dark matter than visible matter.

12. Continued Expansion • Astronomers don’t know what dark matter is made of or how it is distributed through the universe. • Much of the mass of the universe may be dark matter. • In the past few years, astronomers have discovered that the rate of expansion of the universe may be increasing. • Galaxies appear to be moving apart faster now than expected. The reason for this is uncertain. • Dark energy is theorized to be causing the rate of expansion to increase. • If expansion is accelerating, it’s likely that the universe will expand forever.

13. Assessment Questions • According to Hubble’s Law, • the apparent wavelength of light is dependent on relative motion compared to the observer. • the amount of matter in a large region of the universe is constant over long periods of time. • the age of a star is proportional to the red shift of its spectrum. • galaxies are moving away from Earth at speeds proportional to their distance from Earth. ANS: D

14. Assessment Questions • How is dark matter in the universe detected? • direct observation • its effect on electromagnetic interactions • its gravitational effects on matter • very faint background radiation in the universe ANS: C

15. Assessment Questions 3. Based on the size of the universe and its rate of expansion, the big bang is calculated to have occurred between 13 and 14 billion years ago. TrueFalse ANS: T

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